1. Field of the Invention
The present invention relates to a lighting device and a projection type image display system such as a projection television set, an overhead projector and the like. More particularly, it relates to a projection type image display system having microlenses for converging light emitted by the lighting device on respective pixels in a display panel.
2. Description of the Related Art
FIG. 16 shows a structure for a conventional projection type image display system. As is shown in FIG. 16, the projection type image display system has a lighting device comprising a light source 101 for emitting light for illumination and a parabolic mirror 102 for reflecting the light emitted by the light source 101 in a determined direction. A light emitting portion 101a of the light source 101 is positioned at the focal point of the parabolic mirror 102 that is on the optical axis 109. A transmissive type liquid crystal display element 104 as a lighted portion, a condenser lens 105, a projection lens 106 and a screen 107 are provided in this order from the side of the light source 101 so that the central axis of each of these components corresponds to the optical axis 109.
In the above-mentioned projection type image display system, the light emitted by the light source 101 is reflected by the parabolic mirror 102 and changed into the shape of parallel rays to enter the liquid crystal display element 104. An image is formed by the liquid crystal display element 104, and then, the formed image is projected on the screen 107 through the condenser lens 105 and the projection lens 106.
In the transmissive type liquid crystal display element 104 used in the above-mentioned projection type image display system, cross talk is prevented by providing a non-linear element such as a varister and an MIM (metal-insulator-metal) to each pixel in the display panel. Alternatively, each pixel is independently driven by a switching element such as a transistor and the like provided to each pixel. However, when a non-linear element or a switching element is used in the pixel, the pixel has a larger portion which does not contribute to the display, thereby darkening the display and reducing the ratio of aperture. The ratio of aperture herein is defined as a ratio of the total effective area of all the pixels to the area of the display region.
A pixel should be minimized in order to provide a display panel reproducing a desired highly precise image. If the entire components of the pixel were minimized similarly to the pixel, the minimization would not change the ratio of aperture. But a minimization in the width of a metal bus line for electrodes and the size of additional elements such as a non-linear element and a switching element is limited to some extent. This is because accuracy in etching and aligning in the production process is limited. Therefore, as a pixel is minimized, a ratio of the area occupied by a metal bus line for electrodes and additional elements to the entire area of the pixel is increased, thereby decreasing the ratio of aperture.
A low ratio of aperture indicates that more light is blocked by the opaque portion in the pixel which does not contribute to the display. Therefore, a display with a low ratio of aperture looks darker as compared with a display with a higher ratio of aperture even if the same lighting device is used.
In order to prevent the decrease in the ratio of aperture caused by such an attempt for attaining a highly precise image, a microlens array is formed on one surface of the liquid display panel (Japanese Laid-Open Patent Nos. 60-165621 through 60-165624, 60-262131 and 61-11788).
FIG. 17 shows a sectional view of such a liquid crystal display element having a microlens array on a liquid crystal display panel. The liquid crystal display element has a liquid crystal display panel 111 comprising substrates 113 and 114 opposing each other and sandwiching a liquid crystal layer 112 therebetween. A color filter 115 is formed on the inner surface of the substrate 113. A microlens array 116 is provided on the outer surface of the substrate 114 so that each microlens 116a in the microlens array 116 corresponds to each pixel.
In the above described liquid crystal display element, since incident light proceeds along an optical path 121 as is shown in FIG. 18, the light can be converged on an aperture 123 of the pixel satisfactorily by providing the microlens array 116. On the contrary, in the liquid crystal display element without a microlens array 116, incident light proceeds along an optical path 122 to reach other portions than the aperture 123. As a result, a compact and a bright display is attained in the liquid crystal display element having the microlens array 116.
In a commercially available projection type image display system, a light source having a parabolic mirror as is shown in FIG. 16, and a light source as a combination of an elliptical mirror or a spherical mirror and a condenser lens are used in the lighting device. In such a lighting device, light emitted by the light source is reflected by a reflecting mirror such as a parabolic mirror, an elliptical mirror or a spherical mirror in a determined direction.
FIG. 19 is a diagram of a lighting device using a parabolic mirror. In this lighting device, a light source 131 is disposed on the focal point F of a parabolic mirror 132. Light emitted by the light source 131 is reflected by the parabolic mirror 132 and changed into parallel rays shown with solid lines in FIG. 19 and enters a transmissive type display element (not shown).
FIG. 20 is a diagram of a lighting device using an elliptical mirror. In this lighting device, a light source 141 is disposed on the first focal point F1 of an elliptical mirror 142 and a condenser lens (not shown) is disposed on the other side of the light source 141 with respect to the second focal point F2 of the elliptical mirror 142 so that the focal point of the condenser lens corresponds to the second focal point F2 of the elliptical mirror 142. In such a lighting device, light emitted by the light source 141 is reflected by the elliptical mirror 142 to be converged on the second focal point F2, changed into parallel rays and enters the transmissive type display element (not shown).
When a microlens array is used to improve a substantial ratio of aperture as described above, it is required to increase a degree of parallelization of the incident light into the microlens array so as to allow the effect of the microlens array to be exhibited sufficiently. However, in the conventional lighting device, the degree of parallelization is limited due to the arc length of a lamp used as a light source and the accuracy of the reflecting mirror.
When the parabolic mirror as shown in FIG. 19 is used, the light reflected by the parabolic mirror 132 becomes perfect parallel rays as shown with the solid lines if the light source 131 is a complete point source and the parabolic mirror 132 is an idealistic one. However, the arc of the light source 131 actually has a certain size, resulting in producing light as shown with broken lines and dashed lines in FIG. 19. Thus, the degree of parallelization of incident light into the microlens array is decreased. When the arc length is long, an angle .alpha.1 formed by the broken line and the solid line and an angle .alpha.2 formed by the dashed line and the solid line become large, thereby further degrading the degree of parallelization. In addition, since the accuracy of the parabolic mirror 132 is also limited, the light reflected by the parabolic mirror 132 can not be changed into perfect parallel rays even if the light source 131 is a perfect point source.
This also applies to a case where the elliptical mirror as shown in FIG. 20 is used. If the light source 141 is a perfect point source, the light reflected by the elliptical mirror 142 is accurately converged on the focal point F2. But actually, the light can not be completely converged on the second focal point F2 due to the arc length of the light source 141 and the inaccuracy of the elliptical mirror 142. When the arc length of the light source 141 is long, light shown with broken lines and dashed lines is produced, thereby enlarging the converging spot. Thus, the degree of parallelization of the light having passed through the condenser lens is degraded.
FIG. 21 is a diagram of a case where light with a low degree of parallelization enters a microlens. As is shown in FIG. 21, when light enters a microlens 116a having a focal length f at an angle of .+-..theta., a converging spot away from the microlens 116a by the focal length f has a diameter of (2f.times.tan.theta.). The microlens 116a is provided so as to position its focal point on the aperture of a pixel of a display panel. The converging spot does not completely fall on the aperture of the pixel if the value obtained by (2f.times.tan.theta.) exceeds the diameter of the aperture of the pixel. This results in insufficient effect attained by the microlens 116a.
There is another problem as follows in such a case: As is shown in FIG. 16, the light source 101 has shading portions 101b such as an electrode and a lead. When the light source 101 is disposed so that the light emitting portion 101a such as the arc corresponds to the optical axis 109, the light shown with two-dot chain lines does not reach the display element 104 due to the shade of the shading portions 101b of the light source 101. This also applies to the lighting devices as shown in FIGS. 19 and 20. As a result, the subjected image has a dark spot in its center. In this case, in the image projected on a projection screen, illuminance is uneven and a uniform image can not be displayed.
In order to avoid this dark spot, the light source can be shifted toward a lighted portion, i.e., the liquid crystal display element 104 in FIG. 16, away from the reflecting mirror. Then, the dark spot can be avoided, but the light illuminating the center of the lighted portion is not parallel to the optical axis, and too much light is converged on the center of the lighted portion, causing too much difference in illuminance between the center and the periphery of the lighted portion. Such a problem is more severe when the lighted portion is nearer to the light source and the reflecting mirror.
Among the projection type image display systems using the lighting device having the above-mentioned disadvantages, especially the projection type liquid crystal image display system is affected by the degree of the parallelization of the light, since the liquid crystal has high viewing angle dependency. Further, the characteristics of the liquid crystal become ununiform by an uneven temperature distribution on a face parallel to the display surface caused by the irregularity of illuminance distribution, thereby making any projected image uneven.
In a projection type liquid crystal display element having a color filter, the temperature of the substrate bearing the color filter is rapidly raised because the color filter absorbs the light. Moreover, since the illuminance distribution of the light is uneven, the temperature rise becomes also uneven, resulting in causing an irregularity in the temperature distribution on the surface of the substrate bearing the color filter. As a result, an anisotropic stress is caused in the substrate by the temperature irregularity, and this anisotropic stress causes an optical anisotropy, so-called photoelasticity. In a liquid crystal display element using polarization, retardation by birefringence is affected and changed by the photoelasticity. Therefore, contrast is partly degraded, thereby producing a bad effect on the display quality.
The color of the color filter fades with time since it is always exposed to a strong light. The extent of the fading depends upon the strength of the light illuminating the color filter. Therefore, when the illuminance distribution is uneven, the fading is also uneven, which can cause color irregularity.
In a projection type image display system using a liquid crystal display element with a microlens array, the defects of the lighting device results in disadvantages. When a light entering a microlens has a low degree of parallelization, the converging efficiency of the microlens is degraded. Therefore, in the center of an image projected by light with a low degree of parallelization, light having passed through the microlens has difficulties in being converged on the aperture of the pixel as compared with in the periphery of the lighted portion. In order to compensate for this, the light is excessively concentrated in the center of the image as described above. As a result, the photoelasticity due to the uneven illuminance distribution is further accelerated, thereby degrading the display quality.
In a display system using the color filter and the microlens array, the above-mentioned defects of each component are accumulated. As a result, the projected image is uneven and color irregularity is caused, thereby largely degrading the display quality.